As an adjunct to the diagnosis and treatment of disease, the medical industry commonly employs various types of particle flow cytometers, such as that diagrammatically illustrated at 10 in FIG. 1, to analyze particles in a patient's body fluid (e.g., blood cells). For analyzing a patient's blood, for example, a whole blood sample is initially diluted with a saline solution, lysed to explode all the red cells, and then stabilized to return the remaining white cells to their original size.
The prepared blood sample is then placed in a sample holding chamber 12, and a stream of the blood sample is conveyed along a flow channel 11 from the holding chamber 12 through a restricted orifice or aperture 14, that allows particles to be counted one at the time, and into a receiving chamber 16. Via electrodes 21 and 23 that are respectively coupled to either end of the flow cell's holding chambers (holding chamber 12 and receiving chamber 16) a DC electrical field for measuring the displaced volume of each particle and an RF field for measuring the density of each particle passing through the aperture 14 are applied to the flow cell 10 by way of an oscillator-detector circuit 17, which is preferably configured as a Hartley oscillator (although other oscillator architectures may also be used).
As particles pass through the flow cell orifice 14, they introduce changes in the resistance of the orifice in proportion to their size or volume. These changes in resistance are reflected as DC voltage pulses at the electrodes 21 and 23. The density or opacity of the blood cells is associated with changes in reactance of the flow cell aperture 14. By coupling the electrodes 21 and 23 of the flow cell 10 in parallel with the resonance (LC tank) circuit of the RF oscillator-detector circuit 17, changes in the reactance of the flow cell are reflected as a corresponding change in the operation of the RF oscillator, which is measured by means of an RF pulse detector/demodulator.
For non-limiting examples of U.S. Patent literature detailing conventional electronic tube based flow cell RF oscillator detector circuits, attention may be directed to the Coulter et al, U.S. Pat. No. 3,502,974: Groves et al, U.S. Pat. No. 4,298,836; Groves et al, U.S. Pat. No. No. 4,525,666; and Coulter et al, U.S. Pat. No. 4,791,355.
Now although a tube-based flow cell measurement circuit of the type shown in FIG. 1 is effective to provide an indication of both particle size and density, it suffers from a number of problems which are both costly and time-consuming to remedy. A fundamental shortcoming is the fact that it was originally designed as and continues to be configured using relatively old electronic tube components. This potentially impacts component availability, as the number of manufacturers of vacuum (as well as gas filled) electronic tubes continues to decline. In addition, the effective lifetime of a newly purchased and installed tube in the RF (Hartley) oscillator is not only unpredictable, but experience has shown that the effective functionality of most tubes within the Hartley oscillator--detector circuit is very limited, (even though a tube tester transconductance measurement shows a tube to be good). At best a tube can expect to last somewhere in a range of three to nine months--and typically involves on the order of two repair/maintenance service calls per year per flow cell.